As compared with the joining of different materials, adhesive bonding has the advantage, moreover, that it is able to compensate the differences in deformation behavior and in thermal expansion coefficients between the materials, especially when elastic adhesives are used, and hence actually allows such combinations of materials to be joined.
In the literature there are various examples of elastic adhesives. In recent years, in particular, adhesives based on what are called silane-modified polymers have found widespread application by virtue of their universal usefulness. Many examples in the literature address the formulation of adhesive, adhesive sealant and sealant systems for a multiplicity of applications. Mention may be made here, only by way of example, of specifications WO 2006/136211 A1, EP 1036807 B1 and WO 2010/004038 A1, which set out the fundamental concepts of the formulating technologies and formulating constituents that are customary in the art. The base polymer used is customarily a polyether which has been provided, in different processes, with moisture-crosslinking terminal alkoxysilane groups. This product group includes not only the silylated polyethers marketed by the company Kaneka under the name MS Polymer®, but also the so-called silylated polyurethanes (SPUR® products, for example Desmoseal® S, Bayer Materials Science).
The use of polyether backbones in these products is an advantage primarily on account of their low glass transition temperature and the elastic deformation characteristics which are thereby ensured even at low temperatures. However, the silylated polyethers as described in specifications JP 09012863, JP 09012861 and JP 07062222, in particular, on account of their weak intermolecular interaction under service conditions, and the associated reduced intermolecular transmission of forces, do not possess the optimum profile for use in adhesives or sealants.
Silylated polyurethanes as described in DE 69831518 (WO 98/47939 A1) are clearly at an advantage here, since the urethane functions and the urea functions likewise present in specific products allow a high degree of intermolecular force transmission and hence high strengths on the part of the bonds. Silylated polyurethanes as well, however, are hampered by the problems associated with polyurethane, such as the lack of temperature stability and yellowing stability, for example, and also the UV stability, which for certain applications is not sufficient.
Alongside the disadvantages discussed in the two preceding paragraphs, all of the compounds discussed so far have a further property which in many cases is a disadvantage. All discussed products are based on high molecular mass polyether structures of greater than 4000 g/mol, and hence also entail an increased viscosity. In many cases viscosity is so high as to hinder formulation of the respective products.
There is a need, therefore, for alkoxysilyl-modified polymers which retain in unrestricted form the above-described advantages of this class of product, but at the same time exhibit a significantly lower viscosity and thus have more advantageous processing qualities.
There has been no lack of attempts to counteract this weakness, particularly in the silylated polyethers, by means of adroit formulation. For instance, the addition of plasticizers to the silylated base polymer, in particular, is a widespread possibility for generating alkoxysilyl-functional polymers of lower viscosity and easier processing qualities. The profile of properties may be modified, moreover, for the use of reactive diluents, as described in WO 2011/000843 A2 (US 2012/108730 A1).
This approach at a solution, however, has found only limited acceptance, since the formulator who formulates the base polymer, through having to add defined components intended to influence the viscosity of the formulation, is robbed of an important degree of freedom—namely that of modifying the free formulation according to his or her wishes.
It was an object of the present invention, accordingly, to prepare alkoxysilyl-modified polymers which even without assistance from further substances, such as plasticizers or reactive diluents, for example, have viscosities lower than those of comparable known alkoxysilyl-modified polymers. A further object of the present invention was to provide a simple process for preparing such compounds, and also the provision of curable compositions based on such base polymers.
This object has been achieved by means of alkoxysilyl-modified polymers with intrinsically reduced viscosity.
For the purposes of this patent application, alkoxysilyl-modified polymers with intrinsically reduced viscosity are those alkoxysilyl-modified polymers which have a reduced viscosity as set against comparable alkoxysilyl-modified polymers known in the prior art, and whose reduced viscosity does not derive from the addition of one or more auxiliary components to the polymer (after its preparation), but instead whose reduced viscosity is brought about intrinsically, in other words “from the inside”. The expression “reduced viscosity” in the context of this patent application embraces all viscosities which are reduced by at least 5%, preferably reduced by at least 10%, based on the viscosity of an alkoxysilyl-modified polymer with intrinsically reduced viscosity when set against comparable alkoxysilyl-modified polymers, under identical measurement conditions. Alkoxysilyl-modified polymers with intrinsically reduced viscosity may be obtained preferably by the process that is described as part of this invention.